(1) Field of the Invention
The present invention relates to image encoding apparatuses and image encoding methods for encoding moving pictures, and in particular to a technique to improve the image quality of decoded moving pictures.
(2) Description of the Related Art
In recent years, technology development and standardization of standards have been actively taking place for techniques e. g. JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group) that are used to compress/encode moving picture data, for the purpose of transmitting and receiving moving pictures with the use of the Internet and mobile phones and saving moving picture data with digital video cameras.
Among methods of compressing/encoding moving picture data are inter-frame prediction encoding and intra-frame prediction encoding. The inter-frame prediction encoding makes use of relativity between sequential frames in a moving picture. The intra-frame prediction encoding makes use of relativity between pixels that are positioned adjacent with each other in a frame. Between these two, Discrete Cosine Transform (DCT) is widely used for the intra-frame prediction encoding.
For the compressing/encoding processing with the use of Discrete Cosine Transform, DC (Direct Current) components of DCT coefficients have been conventionally used; however, in recent years, AC (Alternating Current) components are also used in addition to DC components. The following documents may be referenced for some examples.
ISO/IEC 14496-1 Information Technology—Coding of Audio Visual Objects—Part 1: Systems.
ISO/IEC 14496-2 Information Technology—Coding of Audio Visual Objects—Part 2: Visual.
In the following description, the compressing/encoding with the use of DC components will be referred to as “DC prediction encoding”, whereas the compressing/encoding with the use of both DC components and AC components will be referred to as “AC prediction encoding”.
When an AC prediction encoding method is used, encoding efficiency is improved compared to when a DC prediction encoding method is used, and the image quality of decoded moving pictures becomes extremely higher. There is, however, a problem that a large memory capacity is required for the processing with AC prediction encoding, and the processing load is high. For this reason, it is a reality that prediction encoding with AC coefficients is used only for small images having small amounts of data, and large images are compressed/encoded with the use of prediction encoding with DC coefficients.
As it is apparent from the fact that many different models of devices such as portable phones and digital cameras start having a larger number of pixels, there is a very high demand for making image quality higher. Consequently, there is a need to improve, with appropriate measures, image quality of large images to which a DC prediction encoding method has conventionally been applied.
Further, AC prediction encoding also has problems. For example, since the processing load is high, the use of AC prediction encoding is avoided in cases where realtime-ness is required, and the image quality therefore cannot be improved. Another problem is that when AC prediction encoding is profusely used in order to prioritize image quality, realtime-ness will be lost (e. g. a frame drop will occur). These problems also tell us that there is a need to make image quality higher with appropriate measures.